Numerous complexes of the multifaceted bioregulatory agent, nitric oxide (NO), with nucleophiles have been synthesized in our laboratories during the past year and shown to exhibit many interesting pharmacological properties. They are active vasorelaxants in vivo as well as in the isolated rabbit aorta. Their in vitro vasodilatory potencies correlate strongly (r = 0.995) with the amount of NO they are known, from decomposition studies conducted in simple buffer solutions, to release spontaneously during the time course of the experiment. They inhibit platelet aggregation in human whole blood, in platelet-rich plasma, and in vivo in rabbit with a duration of action that can be predicted from the rate of spontaneous NO release. They stimulate dopamine release in cultured neurons, while a more commonly used NO donor drug, nitroprusside ion, does not, and are thus of considerable interest in neurobiological research. They inhibit proliferation in A375 human melanoma cell cultures. They are also aiding in the exploration of NO's possible practical significance as a genotoxic agent.